In recent years, highly efficient transmission through digitization is achieved in radio communication, such as wideband code division multiple access (W-CDMA). When a scheme of multi-level phase modulation is applied in radio communication, a technique is adopted by which the amplification characteristics of a transmission-use electronic amplifier at the transmission-side are linearized to suppress nonlinear distortion and reduce electrical power leakage to adjacent channels. If electrical power efficiency is improved using an amplifier inferior in linearity, a technique for compensating nonlinear signal distortion is adopted (see, e.g., Japanese Laid-Open Patent Publication No. 2003-347944).
For example, in order to perform distortion compensation according to the state of the transmission signal to correct memory effect, one technique involves selecting a distortion compensating coefficient using a lookup table (LUT) that associates the coefficients with states of a transmission signal at different times (e.g., differences in electrical power, amplitude, etc.).
The conventional techniques, however, pose a problem of difficulty in accurately compensating signal distortions. For example, in mobile communication, such as W-CDMA, a transmitting apparatus uses great electrical power for transmission, which causes significant nonlinear distortion of the transmission signal, raising a side lobe of the frequency spectrum of the transmission signal and thereby, leading to a leak of electrical power of the transmission signal to adjacent channels. Such electrical power leakage creates noise, which deteriorates the communication quality of adjacent channels. If a wideband signal is used as a transmission signal, in particular, a memory effect becomes larger than in the case of using a narrowband signal. A larger memory effect results in difficulty in compensating distortion near the transmission signal caused by the memory effect.
Variations in the state of the transmission signal results in, for example, variations in the extent to which electrical power and amplitude in the transmission signal vary. Consequently, the distribution range of an address on in the LUT becomes narrower according to the state of the transmission signal, in which case the state of an input signal cannot be accurately identified even if the LUT is used, whereby selection of a proper distortion compensation coefficient is impossible and consequently accurate compensation of signal distortion is impossible.